The present invention relates to a method of manufacturing an epitaxial wafer for use in semiconductor devices. More particularly, the invention is directed to a manufacturing method capable of suppressing the formation of defects such as stacking faults and dislocations in an epitaxial layer (hereinafter referred to as simply xe2x80x9cdefects in an epitaxial layerxe2x80x9d) when the epitaxial layer is grown on a wafer sliced from a silicon single crystal doped with nitrogen.
As the integration density of silicon semiconductor integrated circuit devices increases in recent years, a silicon wafer from which devices are formed is subjected to increasingly severe specifications. Since circuits are made finer with increasing integration density, in a device active region wherein a device is formed on a wafer, crystal defects such as dislocations and elemental metal impurities other than a dopant, which increase leakage current and shorten the life of a carrier are subjected to more rigorous limitations than ever before.
Conventionally, a wafer produced by slicing a silicon single crystal obtained through the Czochralski method has been used for a semiconductor device. Generally, this wafer contains oversaturated interstitial oxygen at a concentration of about 1xc3x971018 atoms/cm3. Although oxygen is effective for enhancing the strength of a silicon wafer by preventing generation of dislocations and for providing a gettering effect, oxygen is well known to deposit in the form of an oxide and to induce crystal defects such as dislocations or stacking faults due to heating during production of a device.
However, in a process of device production, a so-called DZ layer (denuded zone) which is free of crystal defects and which has a thickness of about tens of xcexcm is formed near the wafer surface by diffusion of oxygen to the outside, since the wafer is heat-treated at a temperature as high as 1100-1200xc2x0 C. for several hours so as to form a field oxide film through LOCOS (Local Oxidation of Silicon) and a well diffusion layer. The DZ layer serves as a device active region, to thereby provide a reduction in crystal defects.
However, as higher density and larger integration are being called for in fabricating semiconductor devices, a high-energy ion implantation method is used to form a well, and when the device process is carried out at a low temperature equal to or lower than 1000xc2x0 C., oxygen does not diffuse outward sufficiently and thus the satisfactory formation of a denuded zone near the surface is prevented. To overcome this inconvenience, attempts have been made to reduce oxygen content in a wafer, but such attempts have not been successful in perfectly suppressing crystal defects.
Under such circumstances, an epitaxial wafer wherein an epitaxial layer containing substantially no crystal defects is formed on a wafer has been developed and is widely used for highly integrated devices. However, even using such an epitaxial wafer exhibiting a high degree of crystallinity, device characteristics are degraded due to contamination of its epitaxial layer with elemental metal impurities during subsequent device process steps.
The epitaxial layer becomes more susceptible to contamination with elemental metal impurities as the degree of integration becomes higher and thus the process becomes more complicated, bringing about inconvenience that the influence of the contamination becomes graver. The contamination can be eliminated basically by cleaning a process environment and materials used. However, it is extremely difficult to make the device process completely free of contaminants, and thus gettering technology is required as a means for solving this problem. The gettering technology is a means for entrapping contaminant impurity elements entering an epitaxial layer in a region (sink) which is other than a device active region to make the contaminants harmless.
The gettering technology includes intrinsic gettering (hereinafter referred to as simply xe2x80x9cIGxe2x80x9d) wherein impurity elements are entrapped by utilizing oxygen-caused oxide precipitates spontaneously induced during heat treatment in a device process. However, when a wafer is heat-treated at temperatures as high as 1050-1200 xc2x0 C. in an epitaxial step, oxide precipitate nuclei present within the wafer sliced from a silicon single crystal shrink and extinguish, thereby making it difficult to satisfactorily induce oxide precipitates which serve as a gettering source within the wafer in subsequent device process steps. As a consequence, even if the gettering technology is applied, a problem is addressed that a satisfactory IG effect cannot be exerted on metal impurities throughout the process.
To overcome this problem, the present inventors have made it possible to form oxide precipitate nuclei within a wafer by doping a single crystal with nitrogen while the crystal is grown. These oxide precipitate nuclei are hard to extinguish even with a high-temperature heat treatment during epitaxial step. That is, when a silicon single crystal is grown while doped with nitrogen, the thermal stability of oxide precipitate nuclei within the crystal is increased, whereby the oxide precipitate nuclei do not shrink or extinguish even if they are subjected to an epitaxial step Further, the oxide precipitate nuclei which remain present after the epitaxial step has been completed grow from the initial stage of the device process to effectively function as a gettering sink, and thus an IG effect can be expected.
However, as the study has made a progress, it has been found out that the thermally stable oxide precipitate nuclei, which are obtained by doping a crystal with nitrogen and which are hard to extinguish even after a high-temperature heat treatment, tend to easily induce defects in an epitaxial layer. When the stable oxide precipitate nuclei are induced near the wafer surface, defects, such as stacking faults and dislocations, tend to be easily induced in the epitaxial layer, i.e., a device active region, and thus these oxide precipitate nuclei impose the problem of increasing the leakage current and degrading the gate oxide integrity of a device.
The present invention has been made in view of the problem of defects in an epitaxial layer derived from nitrogen doping, and an object of the invention is therefore to provide a method of manufacturing an epitaxial wafer which is capable of sufficiently suppressing defects in the epitaxial layer to, e.g., 0.1 piece/cm2 or less, even if the epitaxial wafer is prepared from a silicon single crystal grown while doped with nitrogen.
An oxidation-induced stacking fault (hereinafter referred to as simply xe2x80x9cOSFxe2x80x9d) is one type of fine crystal defect attributed to contained oxygen. The OSF is a stacking fault formed in the crystal under an oxide film which is formed during a high-temperature oxidation treatment in a device process. The formation of OSF is positively correlated with the amount of oxygen in a crystal, and the OSF develops with oxide precipitates as nuclei. When a wafer sliced from a silicon single crystal grown by the Czochralski method is thermally oxidized at 1000-1200xc2x0 C. for 1 to 20 hours, ring-like distributed oxidation-induced stacking faults (hereinafter referred to as xe2x80x9cOSF ringxe2x80x9d) may sometimes occur around a single crystal pulling axis. The OSF ring is induced under a high-temperature heat treatment from nuclei of stable oxide precipitates which are hard to extinguish even at temperatures as high as 1100xc2x0 C. or more. Even if an epitaxial layer is grown on a wafer containing an OSF ring, the nuclei of the OSF ring region remain unextinguished.
An OSF ring usually has a width of several millimeters to ten-odds millimeters and is sharply demarcated from other regions. The present inventors have verified from their study that the width of the OSF ring can be increased by doping a single crystal with nitrogen during its growth. Further, it has been verified that when the crystal is doped with nitrogen, the thermal stability of the oxide precipitate nuclei can be improved even in regions other than the OSF ring with these oxide precipitate nuclei being so stable as not to be easily shrinkable and extinguishable even after the epitaxial step. These stable oxide precipitate nuclei exhibit an IG effect from the initial stage of the device process as a gettering source.
As described previously, when the thermally stable oxide precipitate nuclei remain present in a region near the wafer surface, defects in an epitaxial layer tend to be easily induced. Defects in the epitaxial layer tend to occur frequently in regions near an OSF ring in a wafer although reasons therefor are not yet clear. Such defects in the epitaxial layer increase leakage current and degrade the gate oxide integrity of a device.
Further, through a detailed evaluation of the behavior of oxide precipitate nuclei obtained by doping a crystal with nitrogen and factors inducing defects in an epitaxial layer, it has been found out that the formation of defects in an epitaxial layer is influenced by factors such as nitrogen and oxygen concentrations in a crystal, pulling conditions for growing the crystal, and heat treatments before forming the epitaxial layer. That is, it has been found out that the OSF density which is responsible for causing defects in an epitaxial layer can be controlled by controlling the nitrogen and oxygen concentrations. Further, by controlling the location where an OSF ring occurs and the pull rate, and by heat-treating a wafer at high temperatures before growing an epitaxial layer, the size of the precipitate nuclei can be reduced to such a degree as not to induce defects in the epitaxial layer.
The present invention has been completed based on the above knowledge and views, and provides as a gist thereof the following methods (1) to (4) of manufacturing an epitaxial wafer:
(1) A method of manufacturing an epitaxial wafer wherein an epitaxial layer is grown on a wafer sliced from a silicon single crystal which is doped with nitrogen and which has an oxygen concentration of 9xc3x971017 atoms/cm3 or less at an OSF ring region (hereinafter referred to as xe2x80x9cfirst methodxe2x80x9d).
(2) A method of manufacturing an epitaxial wafer wherein an epitaxial layer is grown on a wafer sliced from a silicon single crystal which is doped with nitrogen and which is grown in such a manner that an inside diameter of an OSF ring region is located at a position which is 85% or more of a diameter of the wafer (hereinafter referred to as xe2x80x9csecond methodxe2x80x9d).
(3) A method of manufacturing an epitaxial wafer wherein an epitaxial layer is grown on a wafer sliced from a silicon single crystal which is doped with nitrogen at a concentration between 1xc3x971012 atoms/cm3 or more and 1xc3x971014 atoms/cm3 or less and which is grown at a pull rate of 1.2 mm/min or higher (hereinafter referred to as xe2x80x9cthird methodxe2x80x9d).
(4) A method of manufacturing an epitaxial wafer wherein an epitaxial layer is grown on a wafer sliced from a silicon single crystal which is grown while doped with nitrogen at a concentration between 1xc3x971012 atoms/cm3 or more and less than 1xc3x971014 atoms/cm3, the epitaxial layer being grown after the sliced wafer has been heat-treated at a temperature between 1200xc2x0 C. and 1300xc2x0 C. for 1 minute or more (hereinafter referred to as xe2x80x9cfourth methodxe2x80x9d).